With
almost 1 million inhabitants, Cologne (Köln) is Germany's fourth largest
city. In the 1960s, Köln decided to build an underground tram system
(like Stuttgart or Hanover) instead of a full metro (like Nuremberg
or Munich). The first underground stretch opened in 1968. Most tunnel
sections in the city centre were built until 1974, and in the 1980s
and 90s the tunnel network was gradually expanded. Currently the system
includes 44.5 km of metro-like routes, either underground, elevated
(L13) or at grade. This is complemented by 48 km of conventional railway
lines (Lines 16 and 18 to Bonn) and 94 km of tram-like routes. The total
length of the network is 188 km (12/2010).

The
system is a true network with many lines sharing certain stretches.
In 1994 it was decided that the east-west lines shown in red on the
map will not be converted to Stadtbahn operation with high-floor vehicles.
Instead new low-floor trams were acquired which offer the same accessibility
(35 cm). From Dec. 2003, also the "ring lines" (shown in green)
were added to the low-floor network, whereas all other lines will eventually
be converted to Stadtbahn lines similar to the networks in Stuttgart
or Frankfurt with high platforms (90 cm) and mainly separate right-of-way
when running at grade. Most underground stations have been rebuilt with
high platforms in recent years, but some still have low platforms only
(e.g. Appellhofplatz-Zeughaus, Reichenspergerplatz).

In
Aug. 2007, the line layout was simplified, with several line numbers
(6, 8, 17, 19) having disappeared.

From
Breslauer Platz/Hauptbahnhof to Bonner Wall, a 3.9 km
long north-south tunnel is under construction running via Heumarkt,
Severinstraße and Chlodwigplatz. It will be used
by lines 5 and 16. This section is mostly being excavated by tunnel-boring
machines, with some stations being of the tube-type. South of Bonner
Wall, a surface alignment will take line 5 to Arnoldshöhe, whereas
line 16 will rejoin its present alignment along the River Rhine.
After a severe accident in 2009, the construction of this route has
been delayed by many years and by 2013 only short sections have been
brought into service.